The fossils found in South America push back the origins of live birth in reptiles by 90 million years. Mesosaurs were up to 2 meters long and lived during the Permian- being the earliest marine reptiles known. It kinda makes you wonder why modern day sea reptiles such as turtles, sea iguanas and certain Laticauda sea snakes keep laying eggs to these days. (Ichthyosaurs, plesiosaurs and mosasaurs were all seemingly viviparous).

So they found one young mesosaur seperate from any adult, and found the remains of one inside a female.

It could point to them being cannibalistic as easily as them being viviparous, although personally I would lean towards the viviparous theory in the case of large marine reptiles but not with Mesosaurs due to their small size (in relative terms) and the fact their limbs suggest that they mpost likely spent some time on land.

It also possibly begs another question about some of the larger extinct marine reptiles if they were viviparous. Were all of them air breathers? Large marine reptiles nowadays, along with members of the order Crocodilia etc., all are air breathers and all lay eggs.

It also possibly begs another question about some of the larger extinct marine reptiles if they were viviparous. Were all of them air breathers?

I'm pretty sure they were all air breathers XD Although, who knows? Some sea snakes can obtain much of their oxygen from water... so maybe mosasaurs, relatives to sea snakes, could do the same? It would be helpful if they were say, waiting in ambush underwater for some unsuspecting prey to swim about... I think there's one kind of mosasaur, the Goronyosaurus, that shows some adaptations hinting at a sit and wait predatory lifestyle...

But I would expect even these mosasaurs to need surfacing on a regular basis, like the sea snakes themselves.

I insist, the question here is, why did prehistoric sea reptiles become viviparous, but sea turtles and sea iguanas did not? I can imagine sea iguanas are not as ancient, so maybe that's the reason, but, sea turtles have been around since the Jurassic... why didn´t they become viviparous or ovoviviparous during all that time?